Lesson 1: Network Cabling

Building on our understanding of the different network topologies that
connect computers, we focus next on the cables that connect them. In this
lesson, we examine the construction, features, and operation of each type
of cable, and the advantages and disadvantages of each.

After this lesson, you will be able to:

Determine which type of cabling is best for any networking situation.

Define terms related to cabling, such as shielding, crosstalk, attenuation,
and plenum.

Identify the primary types of network cabling.

Distinguish between baseband and broadband transmissions and identify appropriate
uses for each.

Estimated lesson time: 50 minutes

Primary Cable Types

The vast majority of networks today are connected by some sort of wiring
or cabling that acts as a network transmission medium that carries signals
between computers. Many cable types are available to meet the varying needs
and sizes of networks, from small to large.

Cable types can be confusing. Belden, a leading cable manufacturer,
publishes a catalog that lists more than 2200 types of cabling. Fortunately,
only three major groups of cabling connect the majority of networks:

Coaxial cable

Twisted-pair (unshielded and shielded) cable

Fiber-optic cable

The next part of this lesson describes the features and components of these
three major cable types. Understanding their differences will help you
determine which type of cabling is appropriate in a given context.

Coaxial Cable

At one time, coaxial cable was the most widely used network cabling. There
were a couple of reasons for coaxial cable's wide usage: it was relatively
inexpensive, and it was light, flexible, and easy to work with.

In its simplest form, coaxial cable consists of a core of copper
wire surrounded by insulation, a braided metal shielding, and an outer
cover. Figure 2.1 shows the various components that make up a coaxial cable.

The term shielding refers to the woven or stranded metal mesh
(or other material) that surrounds some types of cabling. Shielding protects
transmitted data by absorbing stray electronic signals, called noise,
so that they do not get onto the cable and distort the data. Cable that
contains one layer of foil insulation and one layer of braided metal shielding
is referred to as dual shielded. For environments that are subject
to higher interference, quad shielding is available. Quad shielding
consists of two layers of foil insulation and two layers of braided metal
shielding.

Figure 2.1Coaxial cable showing various layers

The core of a coaxial cable carries the electronic signals that make
up the data. This wire core can be either solid or stranded. If the core
is solid, it is usually copper.

Surrounding the core is a dielectric insulating layer that separates
it from the wire mesh. The braided wire mesh acts as a ground and protects
the core from electrical noise and crosstalk. (Crosstalk is signal
overflow from an adjacent wire. For a more detailed discussion of crosstalk,
see the section Unshielded Twisted-Pair (UTP) Cable, later in this lesson.)

The conducting core and the wire mesh must always be kept separate from
each other. If they touch, the cable will experience a short, and
noise or stray signals on the mesh will flow onto the copper wire. An electrical
short occurs when any two conducting wires or a conducting wire and a ground
come into contact with each other. This contact causes a direct flow of
current (or data) in an unintended path. In the case of household electrical
wiring, a short will cause sparking and the blowing of a fuse or circuit
breaker. With electronic devices that use low voltages, the result is not
as dramatic and is often undetectable. These low-voltage shorts generally
cause the failure of a device; and the short, in turn, destroys the data.

A nonconducting outer shield—usually made of rubber, Teflon, or plastic—surrounds
the entire cable.

Coaxial cable is more resistant to interference and attenuation than
twisted-pair cabling. As shown in Figure 2.2, attenuation is the
loss of signal strength that begins to occur as the signal travels farther
along a copper cable.

Figure 2.2Attenuation causes signals to deteriorate

The stranded, protective sleeve absorbs stray electronic signals so
that they do not affect data being sent over the inner copper cable. For
this reason, coaxial cabling is a good choice for longer distances and
for reliably supporting higher data rates with less sophisticated equipment.

Types of Coaxial Cable

There are two types of coaxial cable:

Thin (thinnet) cable

Thick (thicknet) cable

Which type of coaxial cable you select depends on the needs of your particular
network.

Thinnet CableThinnet cable is a flexible coaxial cable
about 0.64 centimeters (0.25 inches) thick. Because this type of coaxial
cable is flexible and easy to work with, it can be used in almost any type
of network installation. Figure 2.3 shows thinnet cable connected directly
to a computer's network interface card (NIC).

Figure 2.3Close-up view of thinnet cable showing
where it connects to a computer

Thinnet coaxial cable can carry a signal for a distance of up to approximately
185 meters (about 607 feet) before the signal starts to suffer from attenuation.

Cable manufacturers have agreed upon specific designations for different
types of cable. (Table 2.1 lists cable types and descriptions.) Thinnet
is included in a group referred to as the RG-58 family and has 50ohm
impedance. (Impedance is the resistance, measured in ohms, to the
alternating current that flows in a wire.) The principal distinguishing
feature of the RG-58 family is the center core of copper. Figure 2.4 shows
two examples of RG-58 cable, one with a stranded wire core and one with
a solid copper core.

Larger in diameter and rated for higher frequencies than
RG-59, but also used for broadband transmissions

RG-62

ArcNet networks

Thicknet CableThicknet cable is a relatively rigid coaxial
cable about 1.27 centimeters (0.5 inches) in diameter. Figure 2.5 shows
the difference between thinnet and thicknet cable. Thicknet cable is sometimes
referred to as Standard Ethernet because it was the first type of cable
used with the popular network architecture Ethernet. Thicknet cable's copper
core is thicker than a thinnet cable core.

Figure 2.5Thicknet cable has a thicker core
than thinnet cable

The thicker the copper core, the farther the cable can carry signals.
This means that thicknet can carry signals farther than thinnet cable.
Thicknet cable can carry a signal for 500 meters (about 1640 feet). Therefore,
because of thicknet's ability to support data transfer over longer distances,
it is sometimes used as a backbone to connect several smaller thinnet-based
networks.

Figure 2.6 shows a device called a transceiver. A transceiver
connects the thinnet coaxial cable to the larger thicknet coaxial cable.
A transceiver designed for thicknet Ethernet includes a connector known
as a vampire tap, or a piercing tap, to make the actual physical
connection to the thicknet core. This connector is pierced through the
insulating layer and makes direct contact with the conducting core. Connection
from the transceiver to the NIC is made using a transceiver cable (drop
cable) to connect to the attachment unit interface (AUI) port connector
on the card. An AUI port connector for thicknet is also known as a Digital
Intel Xerox (DIX)connector (named for the three companies that
developed it and its related standards) or as a DB-15 connector.

Figure 2.6Thicknet cable transceiver with detail
of a vampire tap piercing the core

Thinnet vs. Thicknet Cable As a general rule, the thicker the
cable, the more difficult it is to work with. Thin cable is flexible, easy
to install, and relatively inexpensive. Thick cable does not bend easily
and is, therefore, harder to install. This is a consideration when an installation
calls for pulling cable through tight spaces such as conduits and troughs.
Thick cable is more expensive than thin cable, but will carry a signal
farther.

Coaxial-Cable Connection Hardware

Both thinnet and thicknet cable use a connection component, known as
a BNC connector, to make the connections between the cable and the
computers. There are several important components in the BNC family, including
the following:

The BNC cable connector Figure 2.7 shows a BNC cable connector.
The BNC cable connector is either soldered or crimped to the end of a cable.

Figure 2.7BNC cable connector

The BNC T connector Figure 2.8 shows a BNC T connector. This connector
joins the network interface card (NIC) in the computer to the network cable.

Figure 2.8BNC T connector

The BNC barrel connector Figure 2.9 shows a BNC barrel connector.
This connector is used to join two lengths of thinnet cable to make one
longer length.

Figure 2.9BNC barrel connector

The BNC terminator Figure 2.10 shows a BNC terminator. A BNC terminator
closes each end of the bus cable to absorb stray signals. Otherwise, as
we saw in Chapter 1, "Introduction to Networking,"
the signal will bounce and all network activity will stop.

Figure 2.10BNC terminator

NOTE
The origin of the acronym "BNC" is unclear, and there have been
many names ascribed to these letters, from "British Naval Connector" to
"Bayonet Neill-Councelman." Because there is no consensus on the proper
name and because the technology industry universally refers to these simply
as BNC-type connectors, in this book we will refer to this family of hardware
simply as BNC.

Coaxial-Cable Grades and Fire Codes

The type of cable grade that you should use depends on where the cables
will be laid in your office. Coaxial cables come in two grades:

Polyvinyl chloride (PVC) grade

Plenum grade

Polyvinyl chloride (PVC) is a type of plastic used to construct
the insulation and cable jacket for most types of coaxial cable. PVC coaxial
cable is flexible and can be easily routed through the exposed areas of
an office. However, when it burns, it gives off poisonous gases.

A plenum is the shallow space in many buildings between the false
ceiling and the floor above; it is used to circulate warm and cold air
through the building. Figure 2.11 shows a typical office and where to use—or
not use—PVC and plenum-grade cables. Fire codes give very specific instructions
about the type of wiring that can be routed through this area, because
any smoke or gas in the plenum will eventually blend with the air breathed
by everyone in the building.

Figure 2.11Plenum-grade cabling is required
in the plenum by fire codes

Plenum-grade cabling contains special materials in its insulation and
cable jacket. These materials are certified to be fire resistant and produce
a minimum amount of smoke; this reduces poisonous chemical fumes. Plenum
cable can be used in the plenum area and in vertical runs (for example,
in a wall) without conduit. However, plenum cabling is more expensive and
less flexible than PVC cable.

NOTE
You should consult your local fire and electrical codes for specific
regulations and requirements for running networking cable in your office.

Coaxial-Cabling Considerations

Consider the following coaxial capabilities when making a decision about
which type of cabling to use.

Use coaxial cable if you need a medium that can:

Transmit voice, video, and data.

Transmit data for greater distances than is possible with less expensive
cabling.

Offer a familiar technology with reasonable data security.

Twisted-Pair Cable

In its simplest form, twisted-pair cable consists of two insulated
strands of copper wire twisted around each other. Figure 2.12 shows the
two types of twisted-pair cable: unshielded twisted-pair(UTP)
and shielded twisted-pair(STP) cable.

Figure 2.12Unshielded twisted-pair and shielded
twisted-pair cables

A number of twisted-pair wires are often grouped together and enclosed
in a protective sheath to form a cable. The total number of pairs in a
cable varies. The twisting cancels out electrical noise from adjacent pairs
and from other sources such as motors, relays, and transformers.

Unshielded Twisted-Pair (UTP) Cable

UTP, using the 10BaseT specification, is the most popular type of twisted-pair
cable and is fast becoming the most popular LAN cabling. The maximum cable
length segment is 100 meters, about 328 feet.

Traditional UTP cable, as shown in Figure 2.13, consists of two insulated
copper wires. UTP specifications govern how many twists are permitted per
foot of cable; the number of twists allowed depends on the purpose to which
the cable will be put. In North America, UTP cable is the most commonly
used cable for existing telephone systems and is already installed in many
office buildings.

Figure 2.13UTP cable

The 568A Commercial Building Wiring Standard of the Electronic Industries
Association and the Telecommunications Industries Association (EIA/TIA)
specifies the type of UTP cable that is to be used in a variety of building
and wiring situations. The objective is to ensure consistency of products
for customers. These standards include five categories of UTP:

Category 1 This refers to traditional UTP telephone cable that can
carry voice but not data transmissions. Most telephone cable prior to 1983
was Category 1 cable.

Category 2 This category certifies UTP cable for data transmissions
up to 4 megabits per second (Mbps). It consists of four twisted pairs of
copper wire.

Category 3 This category certifies UTP cable for data transmissions
up to 16 Mbps. It consists of four twisted pairs of copper wire with three
twists per foot.

Category 4 This category certifies UTP cable for data transmissions
up to 20 Mbps. It consists of four twisted pairs of copper wire.

Category 5 This category certifies UTP cable for data transmissions
up to 100 Mbps. It consists of four twisted pairs of copper wire.

Most telephone systems use a type of UTP. In fact, one reason why UTP is
so popular is because many buildings are prewired for twisted-pair telephone
systems. As part of the prewiring process, extra UTP is often installed
to meet future cabling needs. If preinstalled twisted-pair cable is of
sufficient grade to support data transmission, it can be used in a computer
network. Caution is required, however, because common telephone wire might
not have the twisting and other electrical characteristics required for
clean, secure, computer data transmission.

One potential problem with all types of cabling is crosstalk. Figure
2.14 shows crosstalk between two UTP cables. (As discussed earlier in this
lesson, crosstalk is defined as signals from one line interfering with
signals from another line.) UTP is particularly susceptible to crosstalk,
but the greater the number of twists per foot of cable, the more effective
the protection against crosstalk.

Figure 2.14Crosstalk occurs when signals from
one line bleed into another line

Shielded Twisted-Pair (STP) Cable

STP cable uses a woven copper-braid jacket that is more protective and
of a higher quality than the jacket used by UTP. Figure 2.15 shows a two-twisted-pair
STP cable. STP also uses a foil wrap around each of the wire pairs. This
gives STP excellent shielding to protect the transmitted data from outside
interference, which in turn allows it to support higher transmission rates
over longer distances than UTP.

Figure 2.15STP cable

Twisted-Pair Cabling Components

While we have defined twisted-pair cabling by the number of twists and
its ability to transmit data, additional components are necessary to complete
an installation. As it is with telephone cabling, a twisted-pair cable
network requires connectors and other hardware to ensure proper installation.

Connection hardware Twisted-pair cabling uses RJ-45 telephone
connectors to connect to a computer. These are similar to RJ-11 telephone
connectors. An RJ-45 connector is shown in Figure 2.16. Although RJ-11
and RJ-45 connectors look alike at first glance, there are crucial differences
between them.

The RJ-45 connector is slightly larger and will not fit into the RJ-11
telephone jack. The RJ-45 connector houses eight cable connections, while
the RJ-11 houses only four.

Figure 2.16RJ-45 connector and jack

Several components are available to help organize large UTP installations
and make them easier to work with. Figure 2.17 shows various twisted-pair
cabling components.

Distribution racks and rack shelves Distribution racks and rack
shelves can create more room for cables where there isn't much floor space.
Using them is a good way to organize a network that has a lot of connections.

Expandable patch panels These come in various versions that support
up to 96 ports and transmission speeds of up to 100 Mbps.

Jack couplers These single or double RJ-45 jacks snap into patch
panels and wall plates and support data rates of up to 100 Mbps.

Wall plates These support two or more couplers.

Figure 2.17Various twisted-pair cabling components

Twisted-Pair Cabling Considerations

Use twisted-pair cable if:

Your LAN is under budget constraints.

You want a relatively easy installation in which computer connections are
simple.

Do not use twisted-pair cable if:

Your LAN requires a high level of security and you must be absolutely sure
of data integrity.

You must transmit data over long distances at high speeds.

Fiber-Optic Cable

In fiber-optic cable, optical fibers carry digital data signals
in the form of modulated pulses of light. This is a relatively safe way
to send data because, unlike copper-based cables that carry data in the
form of electronic signals, no electrical impulses are carried over the
fiber-optic cable. This means that fiberoptic cable cannot be tapped, and
its data cannot be stolen.

Fiber-optic cable is good for very high-speed, high-capacity data transmission
because of the purity of the signal and lack of signal attenuation.

Fiber-Optic Cable Composition

An optical fiber consists of an extremely thin cylinder of glass, called
the core, surrounded by a concentric layer of glass, known as the
cladding.
The fibers are sometimes made of plastic. Plastic is easier to install,
but cannot carry the light pulses for as long a distance as glass.

Because each glass strand passes signals in only one direction, a cable
includes two strands in separate jackets. One strand transmits and one
receives. A reinforcing layer of plastic surrounds each glass strand, and
Kevlar fibers provide strength. See Figure 2.18 for an illustration of
fiber-optic cable. The Kevlar fibers in the fiber-optic connector are placed
between the two cables. Just as their counterparts (twisted-pair and coaxial)
are, fiber-optic cables are encased in a plastic coating for protection.

Figure 2.18Fiber-optic cable

Fiber-optic cable transmissions are not subject to electrical interference
and are extremely fast, currently transmitting about 100 Mbps with demonstrated
rates of up to 1 gigabit per second (Gbps). They can carry a signal—the
light pulse—for many miles.

Fiber-Optic Cabling Considerations

Use fiber-optic cable if you:

Need to transmit data at very high speeds over long distances in very secure
media.

Do not use fiber-optic cable if you:

Are under a tight budget.

Do not have the expertise available to properly install it and connect
devices to it.

NOTE
Pricing for fiber-optic cable is competitive with high-end copper
cabling. Fiber-optic cable has become increasingly easier to work with,
and polishing and terminating techniques now require fewer parts and less
expertise than just a few years ago.

Signal Transmission

Two techniques can be used to transmit the encoded signals over cable:
baseband and broadband transmission.

Baseband Transmission

Baseband systems use digital signaling over a single channel. Signals
flow in the form of discrete pulses of electricity or light. Figure 2.19
shows a baseband transmission with a bidirectional digital wave. With baseband
transmission, the entire communication channel capacity is used to transmit
a single data signal. The digital signal uses the complete bandwidth of
the cable, which constitutes a single channel. The term bandwidth
refers to the data transfer capacity, or speed of transmission, of a digital
communications system as measured in bits per second (bps).

Figure 2.19Baseband transmission showing digital
wave

As the signal travels along the network cable, it gradually decreases
in strength and can become distorted. If the cable length is too long,
the received signal can be unrecognizable or misinterpreted.

As a safeguard, baseband systems sometimes use repeaters to receive
incoming signals and retransmit them at their original strength and definition.
This increases the practical length of a cable.

Broadband Transmission

Broadband systems, as shown in Figure 2.20, use analog signaling
and a range of frequencies. With analog transmission, the signals are continuous
and nondiscrete. Signals flow across the physical medium in the form of
electromagnetic or optical waves. With broadband transmission, signal flow
is unidirectional.

Figure 2.20Broadband transmission showing unidirectional
analog wave

If sufficient total bandwidth is available, multiple analog transmission
systems, such as cable television and network transmissions, can be supported
simultaneously on the same cable.

Each transmission system is allocated a part of the total bandwidth.
All devices associated with a given transmission system, such as all computers
using a LAN cable, must then be tuned so that they use only the frequencies
that are within the allocated range.

While baseband systems use repeaters, broadband systems use amplifiers
to regenerate analog signals at their original strength.

In broadband transmission, signals flow in one direction only, so there
must be two paths for data flow in order for a signal to reach all devices.
There are two common ways to do this:

Through mid-split broadband configuration, the bandwidth is divided into
two channels, each using a different frequency or range of frequencies.
One channel transmits signals; the other receives signals.

In dual-cable broadband configuration, each device is attached to two cables.
One cable is used to send, and the other is used to receive.

Increasing Bandwidth Performance

Increasing the speed of data transmission is a priority as network sizes
and data traffic increase. By maximizing the use of the data channel, we
can exchange more data in less time. The most basic form of data or information
transmission is called simplex. This means that data is sent in
one direction only, from sender to receiver. A simplex transmission is
shown in Figure 2.21. Examples of simplex transmission are radio and television.
With simplex transmission, problems encountered during the transmission
are not detected and corrected. Senders cannot even be sure that the data
is received.

Figure 2.21A simplex transmission

In the next level of data transmission, called half-duplex transmission,
data is sent in both directions, but in only one direction at a time. Examples
of technology that uses half-duplex communication are shortwave radio and
walkie-talkies. Figure 2.22 shows a half-duplex transmission. With half-duplex
transmission, you can incorporate error detection and request that any
bad data be resent. Surfing the World Wide Web is a form of half-duplex
data transmission. You send a request for a Web page and then wait while
it is being sent back to you. Most modem connections use half-duplex data
transmission.

Figure 2.22A half-duplex transmission

The most efficient method of transmitting data is to use a full-duplex
transmission, in which data can be transmitted and received at the
same time. A good example is a cable connection that not only allows you
to receive TV channels, but also supports telephone and Internet connection.
A telephone is a full-duplex device because it allows both parties to talk
at the same time. Figure 2.23 shows full-duplex communication. Modems,
by design, are half-duplex devices. They either send or receive data, switching
between transmission mode and receiving mode. You can create a full-duplex
modem channel by using two modems and two telephone lines. The only requirement
is that both computers be connected and configured to support this type
of communication.

Figure 2.23Full-duplex communication

The IBM Cabling System

IBM has developed its own cabling system, complete with its own numbers,
standards, specifications, and designations. Many of these parameters,
however, are similar to non-IBM specifications.

IBM introduced its cabling system in 1984. The purpose of this system
was to ensure that the cabling and connectors would meet the specifications
of their equipment. The IBM specification includes the following components:

Cable connectors

Face plates

Distribution panels

Cable types

The one IBM cabling component that is unique is the IBM connector, which
is different from standard BNC or other connectors. These are IBM Type
A connectors, known elsewhere as universal data connectors. They are neither
male nor female; you can connect one to another by flipping either one
over. These IBM connectors require special faceplates and distribution
panels to accommodate their unique shape.

The IBM cabling system classifies cable into types. For example, in
the IBM system, Category 3 cable (voice-grade UTP cable) is referred to
as Type 3. (Table 2.2 compares the IBM cabling-system type names with standard
cable type names.) The cable definitions specify which cable is appropriate
for a given application or environment. The wire indicated in the system
conforms to American Wire Gauge (AWG) standards.

AWG: The Standard Cable Measurement

Cable measurements are often expressed as numbers, followed by the initials
AWG. (AWG is a measurement system for wire that specifies its thickness.)
As the thickness of the wire increases, the AWG number decreases. Telephone
wire is often used as a reference point; it has a thickness of 22 AWG.
A wire of 14 AWG is thicker than telephone wire, and wire of 26 AWG is
thinner than telephone wire.

Table 2.2IBM Cabling System

IBM type

Standard label

Description

Type 1

Shielded twisted-pair

Two pairs of 22 AWG wires surrounded (STP) cable by an outer
braided shield; used for computers and multistation access units (MAUs)

Type 2

Voice and data cable

A voice and data shielded cable with two twisted pairs of
22 AWG wires for data, an outer braided shield, and four twisted pairs
of 26 AWG wires for voice

Type 3

Voice-grade cable

Consists of four solid, unshielded twisted-pair, 22 or 24
AWG cables

Type 4

Undefined

Type 5

Fiber-optic cable

Two 62.5/125-micron multimode optical fibers—the industry
standard

Type 6

Data patch cable

Two 26 AWG twisted-pair stranded cables with a dual foil
and braided shield

Type 7

Undefined

Type 8

Carpet cable

Housed in a flat jacket for use under carpets; two shielded
twisted-pair 26 AWG cables; limited to one half the distance of Type 1
cable

Type 9

Plenum-grade cable

Fire safe
Two shielded twisted-pair cables

NOTE
A Multistation Access Unit(MAU) is a hub device
in a token-ring network that connects computers in a physical hub-and-spokes
arrangement, but uses the logical ring required in token ring networks.

Selecting Cabling

To determine which cabling is the best for a particular site you need to
answer the following questions:

How heavy will the network traffic be?

What level of security does the network require?

What distances must the cable cover?

What are the cable options?

What is the budget for cabling?

The better the cable protects against internal and external electrical
noise, the farther and faster the cable will carry a clear signal. However,
the better the speed, clarity, and security of the cable, the higher the
cabling cost.

Cabling Considerations

As with most network components, there are trade-offs with the type of
cable you purchase. If you work for a large organization and choose the
least expensive cable, the accountants might initially be pleased, but
you might soon notice that the LAN is inadequate in both transmission speed
and data security.

Which cabling you select will depend on the needs of a particular site.
The cabling you purchase to set up a LAN for a small business has different
requirements from those of a larger organization, such as a major banking
institution.

In the rest of this section, we examine some of the considerations that
affect cabling price and performance.

Table 2.3 provides comparative information on cabling types.

Installation Logistics

How easy is the cable to install and work with? In a small installation
where distances are short and security isn't a major issue, it does not
make sense to choose thick, cumbersome, and expensive cable.

Shielding

The level of shielding required will affect cable cost. Almost every
network uses some form of shielded cable. The noisier the area in which
the cable is run, the more shielding will be required. The same shielding
in a plenum-grade cable will be more expensive as well.

Crosstalk

Crosstalk and noise can cause serious problems in large networks where
data integrity is crucial. Inexpensive cabling has low resistance to outside
electrical fields generated by power lines, motors, relays, and radio transmitters.
This makes it susceptible to both noise and crosstalk.

Transmission Rates

Transmission rates are measured in megabits per second. A standard reference
point for current LAN transmission over copper cable is 100 Mbps. Fiber-optic
cable transmits at more than 1 Gbps.

Cost

Higher grades of cables can carry data securely over long distances,
but they are relatively expensive; lower-grade cables, which provide less
data security over shorter distances, are relatively inexpensive.

Signal Attenuation

Different cable types have different rates of attenuation; therefore,
cable specifications recommend specific length limits for the different
types. If a signal suffers too much attenuation, the receiving computer
will be unable to interpret it. Most networks have error-checking systems
that will generate a retransmission if the signal is too weak to be understood.
However, retransmission takes time and slows down the network.

Exercise 2.1: Case Study Problem

You have been asked to review the proposals submitted by a consulting firm
to design the cabling scheme for your company's new office building. Table
2.4 and the following diagram illustrate your company's cabling needs.

Table 2.4Your Company's Cabling Needs

Location

Distance

Location

Distance

A to B

15 meters (50 feet)

Hub to A

152 meters (500 feet)

B to C

15 meters (50 feet)

Hub to B

160 meters (525 feet)

C to D

15 meters (50 feet)

Hub to C

168 meters (550 feet)

D to E

61 meters (200 feet)

Hub to D

184 meters (600 feet)

E to F

23 meters (75 feet)

Hub to E

152 meters (500 feet)

F to G

23 meters (75 feet)

Hub to F

130 meters (425 feet)

G to H

23 meters (75 feet)

Hub to G

107 meters (351feet)

H to I

23 meters (75 feet)

Hub to H

91 meters (300 feet)

I to J

61 meters (200 feet)

Hub to I

84 meters (275 feet)

J to K

15 meters (50 feet)

Hub to J

107 meters (351 feet)

K to L

15 meters (50 feet)

Hub to K

99 meters (325 feet)

L to M

15 meters (50 feet)

Hub to L

84 meters (275 feet)

A to M

221 meters (725 feet)

Hub to M

69 meters (226 feet)

D to M

244 meters (800 feet)

A to J

244 meters (800 feet)

The consulting firm has recommended that you implement 10BaseT Category
5 UTP wire for your company's network. Based on this information, answer
the following questions:

Where does this recommendation violate the UTP and 10BaseT specifications?

Lesson Summary

Three primary types of cables are used with networks: coaxial, twisted-pair,
and fiber-optic.

Coaxial cable comes in two varieties: thinnet and thicknet.

Thinnet cable is about 0.64 centimeters thick (0.25 inches) and can carry
a signal for a distance of up to 185 meters (607 feet).

Thicknet cable is about 1.27 centimeters (0.5 inches) in diameter and can
carry a signal for a distance of up to 500 meters (1640 feet).

The BNC connector is used with both thinnet and thicknet cables.

Coaxial cables come in two grades, classified according to how they will
be used: PVC-grade cable is used in exposed areas; plenum-grade cable has
a fire-safety rating and is used in enclosed areas such as ceilings and
walls.

Twisted-pair cable can be either shielded (STP) or unshielded (UTP).

The number of twists per unit of length and the protective shielding provide
protection from interference.

Twisted-pair cables conform to five standards, called categories. Each
category provides specifications for increasing the speed of data transmission
and resistance to interference.

Twisted-pair cables use RJ-45 telephone connectors to connect to computers
and hubs.

Fiber-optic cables use light to carry digital signals.

Fiber-optic cables provide the greatest protection from noise and intrusion.

Data signals can be either baseband or broadband.

Baseband transmission uses digital signals over a single frequency.

Broadband transmission uses analog signals over a range of frequencies.

IBM uses its own system of cabling and standards, but follows the same
basic technology as other cables.